This invention relates to the variable attenuation circuit used for microwave communication device, for example. Microwave communication device is controlling high frequency characteristics, such as a power gain of apparatus and an output power level by using the variable attenuation circuit. As an attenuation circuit, a variable resistor network linked to the T type or I type is constituted, and a diode or a field effect transistor is used as a variable resistor.
However, it is necessary for realizing desired attenuation and desired impedance to decide each resistance of the variable resistor connected in series in parallel as any resistance according to attenuation, when above attenuation circuit is used, and the control circuit setting up resistor of a variable resistance becomes complicated.
Therefore, a circuit using the directivity coupler shown in FIG. 1 or FIG. 2 is used much as a circuit of microwave.
Referring now to the attenuation circuit shown in FIG. 1, a passage terminal 13, and a coupling terminal 14 of a first directional coupler 10 connect to a coupling terminal 24, and a passage terminal 23 of a second directional coupler 20 respectively. Field-Effect-Transistor (FET) 30a, 30b are connected in parallel between the passage terminal 13 and the coupling terminal 24, and the coupling terminal 14 and the passage terminal 23 respectively. An input terminal 11 of the first directional coupler 10, and an input terminal 21 of the second directional coupler 20 are a signal input, and a signal output of this attenuation circuit respectively. And isolation terminals 12 and 22 of respective directional couplers 10 and 20 terminate in termination resistors 15 and 25 respectively.
Signals inputted from the input terminal 11 as signal input are distributed to the passage terminal 13 and coupling terminal 14 by first directional coupler 10. After passing through the parallel circuit of the FETs 30a and 30b, the distributed signals are inputted into the coupling terminal 24 and the passage terminal 23 of the second directional coupler 20 respectively, is compounded, and is outputted from the input terminal 21 of the second directional coupler 20 as signal output. FET 30a and 30b that voltage between drain 31a, 31b and source 32a, 32b is 0 [V] are used as variable resistor by gate bias provided for gate 33a, 33b through resistor 16a, 16b from control terminal 41. The power absorbed by FET 30a and 30b is changed, passage loss is controlled according to changing the resistance of FET 30a and 30b compared with the characteristic impedance (for example, 50 [xcexa9]) of a directional coupler, as a result, the variable attenuation circuit is realized with it.
Moreover, Since the reflective power produced by the mismatching with the impedance of FET and the characteristic impedance of a directional coupler is absorbed by the terminus resistance 15 connected to the isolation terminal 12, it can realize a matching state without returning to the input terminal 11.
Next referring now to the attenuation circuit shown in FIG. 2, this circuit uses the mismatching with impedance of the directional coupler 10 and FET 30a or 30b. And, as a result, a variable attenuation circuit is realized by compounding the reflected signal, making it output from the isolation terminal 12, changing the impedance of FET 30a and 30b, and controlling reflection.
Therefore, in the variable attenuation circuit using the directional coupler, the matching state is realized by using only gate bias of FET connected in parallel as control voltage, and using the character of a directional coupler. In the circuit shown in FIG. 1 as mentioned above, in order to obtain the desired attenuation, the resistance of variable resistor, such as FET, is changed.
However, since reactance component of the impedance by influence of the parasitic capacity of FET or a parasitic inductance becomes large According frequency becomes high, even if gate bias changes, it is not able to change enough in the impedance of FET.
Referring now to FIG. 3, FIG. 3 is a passage characteristic diagram of the variable attenuator in consideration of parasitic capacity of FET. FIG. 3 shows the passage characteristic of the case, for example, in the composition of FIG. 1, used High-Electron-Mobility-Transistor (HEMT) that gate length is 0.3 micrometers and gate width is 300 micrometers. Moreover, it uses four fingers Lange couplers of main frequency 25 GHz as directional coupler. There is a problem that the variable range becomes small remarkably, in high frequency domain, as passage loss becomes large.
Accordingly, it is an object of the present invention to provide a microwave variable attenuation circuit. in the variable attenuation circuit using a coupler, preventing increase of the passage loss and decrease of the variable attenuation by the parasitic capacity of a variable resistor, and having good transmission characteristic.